Growing environmental awareness has developed a market for recycled plastic articles. Recycling of plastic articles is desireable because they are made from nonrenewable petrochemical resources, consume diminishing landfill space, and decompose very slowly. The market for recycled plastic is cost-sensitive, and removing contaminants from different types of post-consumer plastics is a major cost of processing them. Accordingly, high-speed, automated sorting systems are needed to sort foreign materials, including different types of plastics, from post-consumer plastic articles.
Many post-consumer plastic articles are containers, such as beverage containers, that are of a single plastic, such as polyvinyl chloride (PVC) or polyethylene terephthalate (PET). The wide availability of PVC and PET as post-consumer materials make them relatively desireable for recycling.
Articles made of PVC and PET have many common characteristics, such as density, optical transmissivity, and color, and are therefore difficult to sort automatically. However, these two plastics are chemically distinct and PVC, in particular, is considered a contaminant of PET when intermixed during recycling. Such contaminants are difficult to remove during the recycling process and, therefore, greatly diminish the value of the recycled PET plastic.
Some post-consumer plastics sorting systems, such as that described in U.S. Pat. No. 5,141,110 of Trischan et al., seek to sort whole post-consumer containers made of PET and PVC. Such systems can suffer from relatively low throughputs and are incapable of removing from the recycled articles attached foreign objects, such as container caps or tops that remain attached by consumers. Moreover, recycling of post-consumer plastic articles typically includes shredding or flaking the items before subsequent processing. Conventional automated sorting systems can have difficulty distinguishing flakes of PVC from PET.